A gas sensor having a sensor element that varies in electric characteristics in response to a concentration of a particular gas component in an exhaust gas has heretofore been used for an air/fuel ratio control of an automotive vehicle. As such a gas sensor is known, for example, such one that is configured to include a cup-shaped sensor element formed of an oxygen ion conductive solid electrolyte, a heater for heating the sensor element, a metallic housing for holding the sensor element, a tubular metallic member provided to a rear end side of the metallic housing, and lead wires electrically connected to the sensor element and the heater and pulled out from an inside of the tubular metallic member to the outside thereof. The lead wires used for the gas sensor are generally formed by covering conductor wires electrically connected to a sensor element and heater by an insulating film.
In such a gas sensor, it is known a technique for fixing, after a cylindrical elastic seal member having lead wire insertion holes for insertion of the above-described lead wires is disposed inside the tubular metallic member, the elastic seal member is fixed to a tubular metallic member by crimping the tubular metallic member radially inward (refer to Patent Document 1 and Patent Document 2). By employing this technique, the air-tightness between the elastic seal member and the tubular metallic member and between the lead wire and the elastic seal member can be higher, and waterproof of the inside of the tubular metallic member can be attained.
Patent Document 1: Unexamined Japanese Patent Publication No. 9-229897
Patent Document 2: Unexamined Japanese Patent Publication No. 9-54063
In the meantime, an attempt for saving space around an exhaust pipe of in an automotive vehicle is being made in recent years so that there may occur such a case in which a freedom concerning a position for installation of a gas sensor on an exhaust pipe is restricted. Due to this, there arises such a case in which at the time the lead wires pulled out of the tubular metallic member after the sensor is installed on the exhaust pipe are connected to an external circuit such as ECU, they need to be bent leftward and rightward at the points adjacent the rear end opening edge portions of the lead wire insertion holes. Further, there is such a case in which the lead wires pulled toward the rear end side of the gas sensor need to be bent by nearly 180 degrees toward the front end side (element side) and connected to the external circuit.
However, at the time the lead wires of the gas sensor configured so that the rear end peripheral edge of the elastic seal member is positioned at the more front end side than the rear end of the tubular metallic member is connected to an external circuit, there may be caused such a problem that the lead wires are damaged when bent leftward and rightward at the points adjacent the rear end opening edge portions of the lead wire insertion holes. Namely, when the lead wires are bent leftward and rightward at the points adjacent the rear end opening edge portions of the lead wire insertion holes, they are brought into contact with an edge or burr existing at the rear end portion of the tubular metallic member such that there is caused a possibility that when a worker for an wiring operation pulls the lead wires under such a condition, the insulating film is rubbed by the edge or burr to be broken or in the worst case the conductor wire is broken. This problem is liable to be caused particularly when the lead wires are bent by nearly 180 degrees as described above.
Thus, it is considered such a countermeasure that the height of the elastic seal member is increased so as to allow the rear end side thereof to protrude from the rear end of the tubular metallic member so that the lead wires do not directly contact the edge or burr of the tubular metallic member even when the lead wires are bent leftward and rightward, for thereby preventing damage of the lead wires.
However, by the investigation conducted by the inventors it was revealed that a countermeasure of making the rear end side of the cylindrical elastic seal member simply protrude from the rear end of the tubular metallic member caused a new problem that when thermal expansion of the elastic seal member occurred during use of the gas sensor at elevated temperature, stress was locally caused at the elastic seal member due to rubbing with the edge or burr of the tubular metallic member, thus causing a crack in the elastic seal member. In the meantime, when a crack is caused in the elastic seal member, the gas sensor cannot maintain the airtightness and waterproof.